This invention relates to radio antenna systems and methods, particularly antenna systems and methods for enabling one antenna to transmit a first frequency radio signal and simultaneously receive a second radio signal of a frequency near the first radio signal frequency.
Both radio navigation and radio communications equipment are often employed in the operation of vehicles, such as boats, automobiles, and airplanes. One common type of radio navigation is Global Positioning System navigation, commonly known as GPS navigation, which works by computing the vehicle""s position based on radio signals encoded with ephemeris data received from multiple satellites. Satellite telecommunications systems provide worldwide wireless two-way telephonic communications.
Given the current low cost of both GPS navigation and satellite telecommunications systems, many marine, automotive, and airborne vehicles carry both systems on-board. Often, the vehicle passengers and crew find simultaneous operation of both systems desirable.
GPS navigation data signals are currently broadcast at a carrier frequency of 1.575 gigahertz (GHz). Two-way satellite telecommunications systems operate in the L-band frequency range. The system operating frequency is system dependent, with each manufacturer""s equipment operating in a frequency range assigned by license. For example, one commercial system operates in the frequency band of 1.616 GHz and 1.625 GHz. Although satellite telecommunications systems transmit and receive at frequencies nearly the same as the GPS operating frequency, the satellite telecommunications broadcasts and transmissions must not interfere with GPS navigation, at least as regards aircraft operating under FAA regulations.
Normally, each system operates a dedicated antenna. Traditionally, the requirement of noninterference is satisfied, first by filtering the satellite telecommunications system transmitter to limit radio frequency (RF) energy in the GPS operating band, and second by attenuating the RF energy produced by the satellite telecommunications system in the GPS operating band by physically separating the GPS and satellite telecommunications system antennas on the host aircraft. Each of these responses is unsatisfactory. The traditional filter capable of satisfying the requirement is large and heavy, both undesirable traits in aircraft equipment. Antenna space on most aircraft is minimal, further limitations on the antenna system, such as a minimum distance between antennas, exacerbates the problem. Antenna space is similarly limited in some marine and automotive applications. Thus, notwithstanding the above considerations in response to the noninterference requirement, employing a single antenna is desirable to both receive GPS navigation signals and to both receive and transmit satellite telecommunications system signals.
Furthermore, a desire toward economy of cost recommends that a single antenna perform for multiple radio systems. Moreover, a desirable antenna is relatively light weight and low profile to minimize drag and maximize cosmetic appearances.
Additionally, a single antenna both transmitting and receiving communication signals in a first radio frequency band and simultaneously receiving a second radio signal having a frequency near the first radio frequency band is desirably versatile enough to be used with a variety of electronic equipment.
U.S. Pat. No. 5,170,493 discloses a single antenna used to both receive relatively low frequency LORAN-C radio signals broadcast at a carrier frequency of 100 kilohertz (kHz) and to transmit or receive relatively high frequency VHF radio signals broadcast at carrier frequencies in the range of 30 megahertz (MHz) up to 300 MHz. U.S. Pat. No. 5,170,493 also describes a number of other systems for employing the same antenna to both receive relatively low frequency radio signals and to transmit or receive relatively high frequency radio signals by reference to Tanner, et al. U.S. Pat. No. 4,268,805; Elliott, U.S. Pat. No. 4,095,229; and Tyrey, U.S. Pat. No. 4,037,177 Powell, et al. U.S. Pat. No. 3,812,494 discloses a system for receiving and transmitting Doppler frequencies and telemetering frequencies simultaneously over the same antenna, as distinguished from GPS and satellite telecommunications system frequencies, and employs an impedance matching network in close proximity with the antenna itself. Duncan, Jr., et al. U.S. Pat. No. 3,217,273 discloses a system for coupling several transmitters and receivers to a single antenna, while avoiding the radiation of spurious signals resulting from intermodulation, and avoiding swamping the receivers. In addition, U.S. Pat. No. 3,739,390 discloses a system using two antennas operating within different frequency ranges but sharing a single coaxial line, the central conductor forming the receiving element of one antenna and the shield forming the receiving element of the other antenna, duplexing circuitry connected to the antenna separates the signals received at the two antennas and applies the separated signals to the appropriate receiver or transmitter. U.S. Pat. No. 5,574,978 discloses an interference cancellation system allowing different transmitting and receiving radios to utilize a single antenna.
Although a number of systems have been developed for employing one antenna to both receive relatively low frequency radio signals and to transmit or receive relatively high frequency radio signals, such systems solve only the problem as relates to signals orders of magnitude different in frequency. Each of the disclosed systems fails to either consider or resolve the problem of using one antenna to both transmit and receive signals in a first radio frequency band and simultaneously receive a second radio signal having a frequency very near the first radio frequency band as presented, for example, by the problem of combining the function of receiving GPS navigation signals in a single antenna with the function of transmitting and receiving satellite telecommunications system signals where the frequencies are separated by approximately 41 MHz or only 3%. Furthermore, the systems disclosed in the prior would not allow sharing of a single antenna with radio systems operating in the frequency ranges used by GPS and satellite telecommunications systems. Nor would the prior art systems allow sharing of a single antenna with two radio systems operating with such a relatively small frequency spread.
This invention provides a method of utilizing a single antenna for both a satellite telecommunications system transceiver and a GPS receiver, the total implementation of which provides less RF loss in the satellite telecommunications system path, and a smaller filter package than traditional implementations.
The present invention overcomes the limitations of the prior art by providing a method and circuit for using one antenna to both transmit and receive signals in a first radio frequency band and simultaneously receive a second radio signal having a frequency very near the first radio frequency band.
According to one aspect of the invention, the circuit receives a radio frequency (RF) signal in a first radio frequency band introduced into the first port of a directional coupler. The directional coupler divides the energy in the received signal into two signals and introduces each of the divided signals into second and third ports of the directional coupler. Band pass filters coupled to each of the second and third ports of the directional coupler reflect any energy in the divided signals which is in frequency bands other than a second different radio frequency band. The directional coupler combines the energy in the reflected signals into a first RF output signal. The received RF signal is either a broadcast signal received at an antenna coupled to the first port of the directional coupler or a transmission signal generated by a transmitter that is coupled to the first port of the directional coupler. According to one embodiment of the invention, the RF energy reflected by the band pass filters coupled to each of the second and third ports of the directional coupler is energy in the L-Band frequency range, the operation band of typical two-way satellite telecommunications systems. For example, RF energy reflected by the band pass filters coupled to each of the second and third ports of the directional coupler is energy in the frequency band of 1.616 GHz and 1.625 GHz, i.e., the operation band of one known two-way satellite telecommunications system. According to another aspect of the invention, a conventional two-way satellite telecommunications systems transceiver is coupled to the directional coupler to generate and receive signals in the L-Band frequency range, for example, in the frequency band of 1.616 GHz and 1.625 GHz. According to yet another aspect of the invention, a band pass filter is coupled between the output of the directional coupler and the transmitter to clean the generated signal of harmonics in the second frequency range and other spurious emissions.
According to another aspect of the invention, the band pass filters coupled to each of the second and third ports of the directional coupler pass any energy in the divided signals which is in the second different radio frequency band. A second coupler coupled to the above band pass filters combines the passed RF energy into a second RF output signal in the second radio frequency band. According to one embodiment of the invention, the RF energy passed by the band pass filters is in the 1.575 gigahertz (GHz) frequency band. According to one embodiment of the invention, the RF energy passed by the band pass filters in the 1.575 gigahertz (GHz) frequency band is received and processed by a conventional Global Positioning System receiver. According to various other aspects of the invention, a low noise amplifier amplifies and a series of one or more band pass filters coupled between the output of the second coupler and the receiver respectively amplify and clean the received signal.